The 4th Leiden Conference on Mononuclear Phagocytes will focus on the role of monocytes and macrophages in the process of inflammation. It will stress the identification of differentiation antigens by monoclonal antibodies, the control of emigration and the molecular biology of endocytosis and recycling. Important secretory products which may be involved in tissue damage, wound healing and tissue remodeling will be delineated. The interaction of monocytes with platelets and the vascular wall will be prominently discussed in terms of coagulation and chronic inflammatory states of the vasculature - e.g., atherosclerosis, diabetes. The determinants of macrophage activation and the cytocidal mechanisms leading to the kill of parasites and bacteria will be outlined. The role of macrophage arachidonate metabolites in acute inflammation and associated smooth muscle effects as well as their importance in the pathogenesis of renal and joint lesions will be discussed.